This invention relates to a method for controlling an automatic transmission of a vehicle whereby speed variation or shifting is carried out in accordance with predetermined speed-variation characteristics that depend on engine load and a vehicle speed, and in particular to a method for preventing the occurrence of creeping.
In a vehicle having mounted thereon an automatic transmission, a creeping phenomenon can occur wherein the vehicle continues to move at low speed when the accelerator pedal is released. Creeping increases at fast idle and consequently a large braking force is required to maintain the vehicle in a stopped condition.
A method of non-creeping control is disclosed in Japanese Published Unexamined Patent Application No. 17757/1986. It is disclosed therein that, when a braking apparatus of a vehicle is operated during idling while substantially stopped, non-creeping control is performed wherein transmission torque applied to an output side of the transmission, is decreased by upshifting the transmission from a first speed to a second speed, thereby restricting creeping. Another method in Japanese Published Unexamined Patent Application No. 244629/1986 is disclosed in which, in carrying out non-creeping control, a clutch is released to stop the transmission of power to the output side, or an engaging force of the clutch is lowered to decrease the transmission torque to the output side.
In the method of the above-described Japanese Published Unexamined Patent Application No. 17757/1986, non-creeping control is stopped when any of the following three conditions is not satisfied:
(1) vehicle speed is below a predetermined value (the vehicle is substantially stopped),
(2) the opening degree of the engine's throttle valve is below a predetermined value (the throttle valve is substantially at idle), and
(3) the braking apparatus is operated.
Therefore, when the braking apparatus is repeatedly engaged and disengaged while running on a traffic congested road, upshifting and downshifting between first and second speed is repeated. When non-creeping control is carried out by means of the clutch, the clutch is repeatedly engaged and disengaged. As a consequence, shocks generated due to speed change or to engagement of the clutch, impair the drivability (maneuverability) of the vehicle.